Embodiments of the present invention relate to a voltage modulator circuit to control light emission for non-invasive timing measurements.
Recent microprocessor designs use a flip-chip assembly to improve power distribution and achieve higher operating frequencies. Debug probing of such devices relies on what is known as Laser Voltage Probing (LVP). However, LVP technology cannot accurately measure edge delays for multi-GHz frequencies and the laser invasiveness is increasing with smaller transistor geometries.
To overcome these problems, methods to translate signal edge timing information into light emission that can be accurately measured using a Time-Resolved Emission (TRE) or InfraRed-Emission Microscope (IREM) have been proposed. These methods are based on the phenomenon that hot electrons in a saturated NMOS transistor (or beacon device) emit infrared radiation both under static bias and switching condition. See T. Eiles, et. al., xe2x80x9cOptical Probing of Flip-Chip Packaged Microprocessorsxe2x80x9d, ISSCC Digest of Technical Papers, pp. 220-221, February 2000, and L. T. Hoe, et. al., xe2x80x9cCharacterization and Application of Highly Sensitive Infra-Red Emission Microscopy for Microprocessor Backside Failure Analysisxe2x80x9d, Proceedings of the 7th IPFA, pp. 108-112, 1999. Thus, as indicated in FIG. 1, infra-red light is emitted from an NMOS transistor 10 when in saturation, i.e., Vds greater than Vgsxe2x88x92Vt.
J.C. Tsang et al., in xe2x80x9cPicosecond hot electron emission from submicron complementary metal oxide semiconductor circuits,xe2x80x9d Appl. Phys. Lett., p.889-891, February 1997 describes using a commonly available, very low noise optical detector such as mercury cadmium telluride detector array, which has good sensitivity in the range of 0.91-1.45 xcexcm, one can measure the emission intensity (Iemission) accurately. The use of light emission for time-dependent analysis is described by Dan Knebel et al. in xe2x80x9cDiagnosis and Characterization of Timing-related Defects by Time-dependent Light Emissionxe2x80x9d, International Test Conference, p. 733-739, August 1998. This paper describes clock skew analysis as one of many potential applications. In addition, it suggests the use of a phase-detector circuit (PFC) to modulate the duration of light pulse as a function of skew.
Thus, as shown in FIG. 2, in the prior art, a Phase-Frequency Comparator (PFC) 11 is used to focus the mode of operation on one particular edge (i.e. rising edge) for which a timing delay xcex94t is to be measured. The PFC is coupled to a saturated NMOS transistor (or beacon device)13 which emits infrared radiation. The radiation is then detected by a photon detector 15, which may be a TRE or IREM as noted above. The resulting measured pulse, has a width representing xcex94t.
However, we have found that this method is limited by a xe2x80x98deadband regionxe2x80x99 where, if the skew is less than the rise/fall time of the clock under test, it will go undetected. A need, therefore, exists for a method and apparatus which overcomes this limitation.